1. Field of the Invention
The present invention relates in general to local area network implementations and more particularly to the Fiber Distributed Data Interface (FDDI) defined by the ANSI X3T9.5 standard.
2. Brief Description of Related Prior Art
The standard which defines FDDI specifies a local area network based on point-to-point fiber optic links arranged to form a ring. Extremely high data rates are involved: The data rate on these links is 100 megabits per second, encoded to run at 125 megabauds. Given the extremely high data rate, the standard specifies only fiber optic media implementations of the network.
Lower speed networking standards have been specified to run on lower cost copper media. For example, IEEE standard 802.5 specifies networks which are similar in concept to the FDDI. These networks run at 4 or 16 megabits per second (8 or 32 megabauds after encoding) on a shielded twisted pair medium. This IEEE standard 802.5, also known as a token passing ring network is also implemented as a series of point-to-point links connected to form a ring. Other standards, such as IEEE standard 802.3 10Base-T, specify an unshielded twisted pair medium for up to 20 megabaud signaling.
Fiber optic media has some desirable aspects for communication systems. For example, fiber optic media provide high security, as compared to copper media with their associated radiated emissions. Fiber media provide capacity for speeds up to gigabauds. Additionally, fiber media can support link distances up to two kilometers. Unfortunately, the costs associated with these media are very high.
Copper media, such as twisted pair, provide a much more cost effective solution for shorter links. Efficiency for short links is extremely important as studies have shown that most links in a typical local area network are less than 100 meters in length. Accordingly, the need to link distances up to two kilometers is often not a significant consideration. The high costs of the fiber media may often be a very great consideration.